Such devices are particularly used in applications in which it is possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions. The present invention focuses on the ability of SiC to withstand more than five time higher fields in the blocking state of a device than Si, so that it is well suited as a material in high power devices operating under conditions where high voltages may occur in the blocking state of the device.
There are different means used until now for terminating silicon junction devices. Use of a certain type of termination technique is highly desirable otherwise the device will have a low breakdown voltage due to peripheral breakdown.
In properly designed high voltage semiconductor devices the breakdown always occurs via the avalanche mechanism, which implicates a sharp increase of reverse current as the field in the device or in a certain portion thereof. Ideally the breakdown should occur homogeneously over the entire device area because only in this case the maximum performance is obtained. If certain regions exist with a local increase of electric field, then the breakdown will always occur via those regions and at a lower voltage than can be obtained in a device with a uniform electric field. The ideal device with perfectly uniform electric field can never exist because such a device must be infinite, whereas real devices have a certain curvature of electric field at the device periphery. The field curvature is known to be a potential cause of early device breakdown and failure.
There are different means used until now for terminating silicon junction devices and different approaches to the device termination. The way to design the termination of a semiconductor device is to numerically simulate the distribution of electric filed near the device edge. The maximum field in the region of the electric field curvature must be as low as possible and, ideally, lower than the electric field in the central region, the bulk electric field. A number of termination techniques is known for silicon device termination based on the simulations. Those termination techniques are also known to work for cubic materials other than silicon, such as germanium or gallium arsenide.
When with silicon carbide an additional problem exists with a strong anisotropy of avalanche breakdown. The breakdown anisotropy means that the avalanche breakdown not only depends on the absolute value of the electric field, but also on the direction of the electric field. The implication of breakdown anisotropy is that the junction termination techniques developed for silicon will may not work for silicon carbide.
One known solution to this problem is to form device mesas with vertical walls by plasma etching and protect them with silicon dioxide. This can potentially eliminate the lateral field components in the device. However, this technique does not give stable results, presumably due to surface irregularities. Another known approach to this problem is to make a planar device and form a leaky resistive layer at the surface. This technique does not entirely eliminate lateral components of electric field, however, it minimizes them because the leaky layer greatly increases the extension of the depletion layer. A disadvantage of this technique is that the current through the leaky near-surface layer is rather high, resulting in unnecessary power losses and overheating of reverse-biased devices, particularly at high reverse bias.